I can go into this in great depth, but I will keep it fairly simple.
If you look back into the evolution of man from an ape to a human, one trait that stands out is this: the occipital bun found on the back of the skull of neanderthals. Occipital bun is a morphological term used to describe a prominent bulge, or projection, of the occipital bone at the back of the skull. The term is most often used in connection with scientific descriptions of classic Neanderthal crania. While common among many of mankind's ancestors, primarily robust relatives rather than gracile, the protrusion is relatively rare in modern Homo sapiens.
Some scientists suspect occipital buns might correlate with the biomechanics of running. This was important to our ancestors, who transitioned from relatively immobile apes who could only run a short distance, to neanderthals who could run great distances. The bun helped them stabilize their head as they ran...think of it as a counterweight.
Now, there are still many human populations which often exhibit occipital buns. A greater proportion of early modern Europeans had them, but prominent occipital buns even among Europeans are now relatively infrequent. They are still found fairly often among Lapp and Finn individuals. Bushmen from South Africa and Australian aborigines often have occipital buns also. A rare occurrence is found in the north of England where the occipital bun is widespread in south Lancashire.
If you are a descendant of one of these peoples, it might be why your head is so damn big.